It is desireable for power supply equipment to be operable from any mains voltage that is available in any country in the world without mechanical switches, adjustments or other user intervention. Accordingly, the power supplies must work with a voltage range from approximately 85 volts A.C. (Japanese low line) to 265 volts A.C. (European high line) and any voltage in between. Conventional power supply designs which meet these requirement require larger or more expensive components, such as transformers, capacitors and switching transistors to accomodate wide variations in mains voltages than would be required for a narrower operating range. Furthermore, the efficiency of a power supply which accomodates mains voltages over a wide range is lower by comparison to power supplies operable over a narrower operating range. While, the narrower operating range is acceptable when it is possible to switch, reconfigure or adjust the power supply for the specific, narrow mains voltage range, such manual alterations are undesireable because the user can easily missconnect the power supply, with catastrophic results.
One approach to provide power supply operation over a wide input range, particularly where the mains voltages comprise two ranges, of approximately 85-132 volts and 180-265 volts, having an approximate 2:1 voltage ratio, is to provide a power supply having a configutation switchable between a first and a second mode. The first mode provides a rectified D.C. output voltage by full wave rectification of the mains power when in the higher voltage range, while the second mode switches the power supply to a voltage doubler when the mains is in the lower voltage range. Previous techniques have provided power supply mode selection in response to a long term average (several cycles) of the input mains voltage, such as determined by the magnitude of the rectified D.C. output voltage. However, the mode control derived from a long term average typically causes the circuit to initially operate in the voltage doubling mode even for mains input in the higher voltage range, subjecting the input of the power supply to unacceptably high voltages during the start-up period. Undesired voltage doubling can also occur during periods of intermittant connection to the mains, which would cause the long term average to drop to a threshold, causing the power supply to operate in the voltage doubler mode. Similarly, a dip in the magnitude of the mains voltage into the lower range for a few cycles followed by a recovery to the higher voltage range would cause the power supply to improperly voltage double, causing an overshoot in the resulting rectified D.C. to exceed the desired range, possibly damaging subsequent circuit elements. Furthermore, slow responding power supply circuits would respond to, and in certain circumstances, magnify the effect of transients imposed on the power mains such as lightning, or other external radio frequency interferrence (RFI). Such signals, if of appropriate magnitude and duration, would cause the slow responding power supply mode control to cause the power supply to switch mode, thereby exacerbating the transients on the mains.
The long term mains voltage variations as may occur during brown-outs, and which would properly permit the circuit to operate in voltage double mode, must nevertheless cause the power supply to immediately be restored to full wave operation when the brown-out is corrected.